Alloying-induced enhancement of thermopower in the Dirac-semimetal system Cd 3 − x Zn x As 2

${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ is one of the prototypical topological Dirac semimetals. Here, we manipulate the band inversion responsible for the emergence of Dirac nodes by alloying ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ with topologically trivial ${\mathrm{Zn}}_{3}{\mathrm{As}}_{2}$. The carrier density monotonically decreases and the resistivity is enhanced as $x$ is increased. For larger $x$, the thermoelectric figure of merit exhibits comparably large values exceeding 0.3 at room temperature, due to the combined effects of a strong enhancement of the thermopower, an only moderate increase in the resistivity, and a suppression of the thermal conductivity. Complementary quantum-oscillation data and optical-conductivity measurements allow us to infer that the Dirac nodes are gapped out, and a band structure with a partly flat dispersion likely emerges in the higher-$x$ region in ${\mathrm{Cd}}_{3\ensuremath{-}x}{\mathrm{Zn}}_{x}{\mathrm{As}}_{2}$.